Electrodialysis apparatus



Sept. 15, 1964 Filed July 26. 1956 A. J. GOTTSCHAL ETAL ELECTRODIALYSIS APPARATUS f P /6 /7 FIG. 1.

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INVENTORS J GOTTSCHAL F SPEARMAN I G. WIECHERS R. WILSON Sept. 15, 1964 A 3,149,062

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' INVENTORS A.J. GOTTSCHAL S. F. SPEARMAN 5.6. WIECHERS J. R. WILSON BY g ft/z Se t. 15, 1964 A. J. GOTTSCHAL ETAL 3,149,062

ELECTRODIALYSIS APPARATUS Filed July 26, 1956 4 Sheet Sheet 3 r INVENTORS A. J. GOTTSCHAL S. F. SPEARMAN S. G. WIECHERS J. R. WILSON P 15, 1964 A. J. GOTTSCHAL ETAL 3,149,062

ELECTRODIALYSIS APPARATUS 4 Sheets-Sheet 4 Filed July 26, 1956 I m y a 1 I V/ 17/4 .74 7A uumL H.. 7/4 ZQE 4 TL H F 6. IO.

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United States Patent 3,149,062 ELECTRODIALYSIS APPARATUS Auko Jan Gottschai, Lakeside, R0. Northrand, Transvaal, and Stanley Frank Spear-man and ybrandus Gerhardus Wiechers, Pretoria, Transvaal, Republic of South Africa, and John Reuel Wilson, The Hague, Netherlands, assignors to South African Council for Scientific and Industrial Research, Pretoria, Transvaal, Republic of South Africa Filed July 26, 1955, Ser. No. 600,328 Claims priority, application Republic of South Africa July 30, 1955 7 Claims. ((11. 204-301) In general, this invention relates to the treatment of liquids by electrodialysis, and in particular relates to apparatus and the arrangement of such apparatus to accomplish electrodialytic processes at a high level of efiiciency.

This invention constitutes an improvement in design and arrangement of known forms of electrodialytic apparatus, for example, such as is disclosed in Union of South African Patents Nos. 19,860 and 20,748, which relate to multicell electrodialytic units formed by alternate anion and cation permeable membranes separated by suitable intermembrane spacers all of which being mounted together to form a press between a pair of backing plates which carry electrodes.

It was found that in such electrodialytic multicell apparatus used in the demineralisation of saline water the coulomb efliciency was not independent of the number of cells and that such coulomb efliciency dropped significantly as the number of cells was increased above about twenty. This decrease in coulomb efiiciency With increased number of cells partially offsets the known advantages of an apparatus consisting of a large number of cells, namely, a small proportion of power dissipation at the electrodes.

An object of this invention is to provide arrangements for electrodialytic processes and more specifically multicell electrodialytic apparatus, having low electrical power requirements and a high effectiveness per unit total membrane surface.

A further object is to provide arrangements of electrodialysis apparatus in which the coulomb efficiency is maintained at a high level even when large numbers of cells, for example, fifty or more, are combined to form the multicell unit.

Another object of this invention is to provide an electrodialysis apparatus wherein external connections to and from the various infiuent and effiuent channels or conduits of the apparatus are easily and readily made with out weakening the arrangement and construction of the unit.

According to this invention, a multicell electrodialysis apparatus, comprising a plurality of fluid spaces formed by alternate anion and cation permeable membrane walls separated by intermediate spacer members all being mounted together to form a press between a pair of backing or end plates carrying electrodes, is characterized in that the electrical resistance of the path through which electric current dissipates, relative to that of the path through the active membrane surfaces, is increased and whereby substantially the total available electric current is employed in effecting electrodialysis.

More particularly one feature of this invention provides, at selected positions in the membrane press, intermediate plates arranged to interrupt the continuity of the electrolyte conduit streams through the membranes without interrupting the electrical continuity at the active membrane surfaces, said intermediate plates serving to subdivide the membrane press into two or more packs.

3,140,062 Patented Sept. 15, 1964 Furthermore, according to this invention the membranes, intermembrane spacers, backing plates and intermediate plates are characterized by having shapes such that the distances from the geometrical centre of the active membrane surface to the conduits or channels containing the electrolyte streams within the press, and more particularly the conduits containing the more concentrated electrolyte (brine) streams, are increased to the limit of practicability, such limit being consistent with maintaining an adequate proportion of active membrane surface, say not less than about 70%.

The introduction of intermediate plates and/or electrically insulating elements at selected positions in the membrane press to interrupt the continuity of the electrolyte conduit streams makes necessary other features according to this invention, namely, arrangements to effect continuity of flow in the electrolyte system. Thus, according to this invention, in a multicell electrodialysis apparatus provided with intermediate plates, continuity of flow of the electrolyte system of the press is preserved by connecting the separated electrolyte streams via external paths of high electrical resistance compared with the electrical resistance of the membrane packs constituting the press, preferably not less than, say, fifty times as high.

A further feature of this invention which is designed to offset the loss in coulomb efficiency inherent in multicell electrodialysis apparatus of the known and conventional type is an arrangement of spacer material and membrane spacers which reduces to a minimum the volume of electrolyte between the membranes.

An additional feature of this invention is the recognition of the importance of employing membranes of high electrical conductivity. Thus, in preferred assemblies according to this invention, the anion and cation permeable membranes constituting the multicell unit are of high electrical conductance and not less than about millimho per square centimetre membrane surface as measured in 1,000 ppm. neutral aqueous NaCl solution at 30 C. at a frequency of 2,000 c.p.s. Use of membranes of high electrical conductance is considered necessary and results in advantages gained in operational efliciency of multicell electrodialysis apparatus over and above that anticipated from a consideration of the effect of the improved membrane conductance upon the overall electrical resistance of the membrane assembly alone. In'fact, the importance of employing membranes of high electrical conductance is such that the advantages gained by the application of the various features of this invention to reduce loss of coulomb efliciency in multicell apparatus are annulled when membranes of low electrical conductance are used. However, in order to arrive at and retain high coulomb efliciency values for multicell electrodialysis units by employing membranes of high electrical conductance, such desired conductance characteristics for the membranes must not be obtained at the expense of the other properties of the membranes such as, for example, permselectivity, permeability to electrolytes and mechanical strength.

The aforesaid features or measures all achieve substantially the same effect, namely, increasing the electrical resistance of the path through which electric current is dissipated relative to that of the path through the active membrane surfaces of the multicell in which the electric current is usefully employed in effecting electrodialysis. This indicates that the lower coulomb efficiencies found with electrodialysis units composed of large, as compared with small, numbers of cells are due to the fact that, with increase in the number of cells, an increasing proportion of the current supplied to the apparatus is short circuited through the electrolyte channels or conduits without making any contribution to the electrodialytic effect.

Thus, the coulomb efficiency of a multicell electrodialysis apparatus may be increased by applying any one of the aforesaid features or embodiments, or combination of such features or embodiments.

In order that the invention may be more clearly understood, and for the purpose of illustration, reference will now be made to the drawings accompanying this specification.

FIGURES 1, 2, 3 and 4 are elevations of four forms of membrane shapes, typical of this invention;

FIGURE 5 is a diagrammatic isometric exploded view showing in some detail the assembly and arrangement of a typical membrane pack and part of an adjacent membrane pack within an electrodialysis press in which membranes of the shape shown in FIGURE 3 are used;

FIGURE 6 is a fragmentary isometric view of two packs of a membrane press in which membranes of the shape shown in FIGURE 4 are used;

FIGURE 7 is a fragmentary isometric view of two packs of a membrane press in which membranes of the shape shown in FIGURE 2 are used;

FIGURE 8 is a diagrammatic fragmentary side view of a typical membrane press constructed according to this invention;

FIGURE 9 is a diagrammatic representation of another arrangement according to this invention showing the flow .of the dialysate and brine streams and the manner in which these streams are connected to the apparatus;

FIGURE 10 is an elevation of an intermediate plate typical of the invention, showing the manner in which electrically conducting electrolyte may be introduced for the purpose of preserving electrical continuity at the active membrane surface of an electrodialysis press; and

FIGURE 11 is a sectional elevation on line XIIXII of FIGURE 10.

Referring to FIGURE 5, reference numeral 1 denotes an end or backing plate in which is set, in the position shown, an electrode 2. The electrical connections to the said electrode and the special electrode rinse compartment normally associated with electrodialysis units are not shown for .the sake of simplicity. The end plate 1 and its attendant electrode 2 constitute one of two end or backing plates terminating a typical electrodialysis membrane press.

Following the end or backing plate 1 and the associated electrode rinse compartment, are arranged in the sequence shown, gaskets 3, sheets of suitable spacer material 4, dialysate and brine distribution rings 5 and '7 respectively, and membranes 6 and 8 which are alternately anionic and cationic permselective. The thickness dimensions of the gaskets, spacer material and rings thus define the width and free volume of the various diluting and concentrating compartments formed by the alternate anion and cation permeable membrane walls.

In this way the whole of the membrane pack 9 is assembled from sequential gaskets, sheets of spacer material, rings and membranes terminating in an intermediate plate Ill. The arrangement of gaskets, spacer material, rings and membranes immediately preceding said intermediate plate may vary according to the design of the latter.

In a like manner, the second membrane pack of the press is built up and assembled from intermediate plates 11 and sequential gaskets, spacer material members, rings and permselective membranes. Each membrane pack within the press is defined by separate or individual pairs of intermediate plates except in the case of the packs situated at the extremities of a press in which case the packs are defined by a single intermediate plate and an end or backing plate carrying an electrode. Intermediate plates relating to adjacent membrane packs, for example, plates It and 11 in FIGURE 5, are separated by a special gasket 3:: as shown.

Referring to FIGURES 5, 6, 7 and 8, brine enters and leaves the first membrane pack via inlets and outlets 13 and I4 situated adjacent the extreme bottom and top ends of the end plate 1 and the intermediate plate 10 while the dialysate enters and leaves the pack via inlets and outlets l5 and lo situated as shown.

Similarly, in the second pack of the press brine and dialysate enter and leave via inlets and outlets situated at the bottom and top of the intermediate plates 11 which define the pack.

Within the membrane pack itself, the brine and dialysate streams flow in parallel through the cells via conduits formed by the holes 17 and I8 (FIGURES 1 to 5) in the corresponding membranes and the respective rings. The holes I? provide the brine channels or conduits whereas the holes 18 form the dialysate conduits or channels.

Referring again to FIGURE 5, reference numeral 12 denotes sheets of spacer material which fit into the re cessed faces of the intermediate plates It) and 11.

FIGURE 8 shows diagrammatically a typical membrane press made up of a number of packs as described above. Said drawing shows a manifold feed and efiiuent arrangement for both brine and dialysate and the way in which the several packs are connected in parallel flow. The pipes 19 and 2t respectively represent the brine and dialysate supply whereas .21 and 22 are respectively the brine and dialysate effluent tubes.

Although FIGURE 8 indicates feed inlets and efiluent outlets in opposite ends of each of the several packs, other arrangements are possible. Brine and dialysate may also be fed at both ends of the bottom of each pack and leave through similarly located corresponding outlets at both ends of the top of such pack.

. A construction of membranes, intermembrane spacers and backing plates as also intermediate plates according to the shape shown in FIGURE 1 results in a large inactive membrane surface and is, therefore, wasteful, whilst constructions according to FIGURES 2, 3 and 4 are economical because such constructions result in a total inactive surface per membrane of bXc only (neglecting the narrow inactive portions at either side of the electrode area which do not vary with construction). The active (electrode) area is shown in FIGURES l, 2, 3 and 4 as the shaded regions. The distance a represents the spacing from the brine conduit or channel to the active surface of a membrane. Such distance should be a maximum within practical limits.

The intermediate plates shown in FIGURE 10, composed of electrically insulating material which interrupts the continuity of the electrolyte channel streams, have openings 3%) in their central region so that the electrical continuity of the membrane press is preserved. Each plate is provided with an inlet 27 and outlet 28 for the conducting electrolyte rinse stream and a recess 31 permitting distribution of said electrolyte. The holes 29 allow for sealing of the entire assembly by means of bolts. Since it is necessary to arrange for continuity of flow in the electrolyte system, the separated electrolyte streams must be connected via paths of high electrical resistance relative to the electrical resistance of the membrane packs, preferably not less than 50 times as high. An arrangement for achieving this is shown in FIGURE 8, employing membranes, intermembrane spacers and intermediate plates having the shape as shown in FIGURE 3. Alternative constructions are possible, such as those having membranes of the shapes shown in FIGURES 2 and 4 but the principle of securing the desired connections is the same.

Referring to FIGURE 8 of the accompanying drawings, the first membrane pack is arranged between end plate 1 and intermediate plate 10 whilst the second pack is arranged between intermediate plates 11 and 11, the plates being reversed so that the dialysate and brine connections of alternate packs are disposed in staggered relationship in order that space is made available for the connections of the pipes to the individual electrolyte channels, for example, the brine pipes 19 and 21 and the dialysate pipes 20 and 22. The external connections of similar streams may be arranged as is shown in FIGURE 9 to provide for packs connected in series; long paths should be employed to the manifolds, more especially in the case of the brine. In FIGURE 9 the brine streams are denoted by double arrows. Other arrangements of flow are possible, for example, the several packs of a membrane press may be arranged in series-parallel in which case the various connections at the top and bottom of the plates will be altered accordingly.

A further improvement is obtained in the multicell electrodialysis apparatus by providing a small volume of liquid between the membranes per square centimetre of membrane surface, so that the electrical resistance of the intermembrane electrolyte in each compartment in a direction parallel with the plane of the membrane is made as high as possible relative to the electrical resistance per unit length in a direction at right angles to the active membrane surface. This may be achieved either by a small membrane spacing or, for example, by use of perforated corrugated intermembrane spacing material of which the thickness of such corrugated material is appreciable in comparison with the amplitude of corrugation. It has been found possible to have intermembrane electrolyte volumes as low as 0.06 ml. per square centimetre membrane surface. A practical limit is set to the extent to which this electrolyte volume can be reduced, by the necessity for the pumping energy requirements not to be unduly high, say about ft. head of water.

In addition to the advantages gained in respect of increased coulomb efficiency, it will be appreciated that major mechanical advantages derive from the adoption of certain constructions according to this invention. For example, the use of intermediate plates, by sub-dividing a multicell apparatus containing a large number of cells into smaller integral assemblies, overcomes at least three serious disadvantages inherent in such large apparatus. These disadvantages are, firstly, a tendency of the membrane stack towards bending when closely clamped thus prejudicing the easy attainment of the water-tight seal necessary in practice, secondly, the difficulty in effecting even distribution of liquid within the packet, and thirdly, maintenance difficulties encountered during the removal and replacement of defective portions of the packs when such is found necessary during the operation of the apparatus.

The addition of intermediate plates to multicell electrodialytic apparatus of the conventional form, such as is shown in FIGURE 1 of the accompanying drawings, makes it necessary for the various liquid streams to be introduced into and removed from the unit via inlets and outlets provided in the sides of the intermediate plates, unless the press consists of two packs, only. It will be clear from FIGURES 5, 6 and 7 that this drawback is eliminated by the adoption of membranes, intermembrane spacers and intermediate plates of such non-rectangular shapes as are for example shown in FIGURES 2, 3 and 4. Thus, the adoption of preferred constructions according to this invention, such as are illustrated in FIGURES 2 to 7 allows convenient and ready assembly and dismantling of large multicell electrodialytic apparats.

The membrane press arranged and described in the drawings represents only typical assemblies for such a unit, but many other features and combinations of features may be introduced into the apparatus without departing from the principles of the invention. For example, special electrode rinse compartments may be introduced at the electrode end or backing plates terminating the press. The use of special rinse liquid streams may be extended to embrace the intermediate plates through which may be made to circulate a separate stream of liquid containing a high concentration of electrolyte for the purpose of maintaining electrical continuity at the active membrane surface.

The form taken by the intermediate plates may also be changed in several respects, for example, instead of forming the end brine compartments of the packs by recessing the faces of the corresponding intermediate plates, as shown in FIGURES 5 and 11, these compartments may be formed by additional gaskets and rings.

In an arrangement of a multicell electrodialysis apparatus, a high voltage drop is experienced across a unit composed of a large number of cells where the total current is supplied by a single pair of end electrodes situated in the backing plates. However, the arrangement of such apparatus according to this invention permits ready application of extra electrodes mounted in one or more of the intermediate plates of such multicells. Thus, for example, in a unit made up of, say, twelve membrane packs, supplementary electrodes could advantageously be mounted at intervals of every four packs so that a lower voltage supply would be required. Such subsidiary electrodes are connected in parallel, possibly with means whereby the voltage over parts of the unit may be adjusted by external resistances for balancing purposes or when it is necessary to replace a faulty pack. Said subsidiary electrodes may be arranged as a common electrode between adjacent intermediate plates, or adjacent intermediate plates may each carry a subsidiary electrode.

In the foregoing, specific disclosures of our invention are made for the purpose of explanation of the broader aspects of our invention but it is to be understood that the details may be modified in various respects without departure from the principles of the invention and that the invention may be applied to and practised on other structures than those described and illustrated.

We claim:

1. An electrodialysis apparatus comprising a plurality of juxtaposed multi-membraned units each conprising a first intermediate plate at one end of the unit and an identical intermediate plate at the other end of the unit rigidly connected to the first intermediate plate, a plurality of parallel alternating anion-selective and cationselective membranes between said intermediate plates, at least one gasket and one intermembrane spacer member between each two membranes defining between said membranes alternate diluting and concentrating compartments having inlets and outlets, all said membranes, gaskets, intermembrane spacer members and intermediate plates being parallel to each other, said intermediate plates being of insulating material and of the same outline as said membranes and gaskets, and having apertures therein in the area thereof which is in contact with the active membrane surfaces of said membranes and having ports therein, said membranes, gaskets, and spacer members having alinged holes therein forming manifold conduit means separately interconnecting the inlets of all the diluting compartments of said unit and separately interconnecting the outlets of all the diluting compartments of said unit, said membranes, spacers and gaskets having further aligned holes therein forming further manifold conduit means within each unit interconnecting the inlets of all the concentrating compartments of each unit and separately interconnecting the outlets of the concentrating compartments of said unit, the ports in said intermediate plates registering with said manifold conduit means, external disconnectable conduits connecting the manifold conduit means for the diluting compartment inlet and outlets of one unit with the manifold conduit means for the diluting compartment inlets and outlets of adjacent units, further external disconnectable conduits connecting the manifold conduit means for the concentrating compartment inlets and outlets of one unit with the manifold conduit means for the concentrating compartments of adjacent units, and electric current supplying means consisting of an anode at one end of the apparatus and a cathode at the other end of the apparatus for passing a direct current transversely through all the membranes and compartments.

2. An electrodialysis apparatus as claimed in claim 1 and common external supply conduit means for diluting fluid, common external supply conduit means for concentrating fluid, common external withdrawal conduit means for diluting fluid and common external Withdrawal conduit means for concentrating fluid, the manifold conduit means Within each unit connected to the inlets to the diluting compartments being connected to said common external supply conduit means for diluting fluid, the manifold conduit means within each unit connected to the inlets to the concentrating compartments being connected to said common external supply conduit means for concentrating fluid, the manifold conduit means Within each unit connectedto the outlets for the diluting compartments being connected to said common external withdrawal conduit means for diluting fluid, and the manifold conduit means within each unit connected to the outlets for the concentrating compartments being connected to the common external Withdrawal conduit means for concentrating fluid, whereby the units are connected in parallel.

3. An electrodialysis apparatus comprising a plurality of juxtaposed multi-membraned units each comprising a first intermediate plate at one end of the unit and an identical intermediatev plate at the other end of the unit rigidly connected to the first intermediate plate, a plurality of parallel alternating anion-selective and cation-selective membranes between said intermediate plates, at least one gasket and one intermembrane spacer member between each two membranes defining between said membranes alternate diluting and concentrating compartments having inlets and outlets, all said membranes, gaskets, intermembrane spacer members and intermediate plates being parallel to each other, said intermediate plates being of insulating material and of the same outline as said membranes and gaskets, and having apertures therein in the area thereof which is in contact with the active membrane surfaces of said membranes and having ports therein, said intermediate plates further having rinse electrolyte ports therein, the apertures in said plate being connected to each other and to said rinse electrolyte ports, said connected apertures forming rinsing compartments, said membranes, gaskets, and spacer members having aligned holes therein forming manifold conduit means separately interconnecting the inlets of all the diluting compartments of said unit and separately interconnecting the outlets or" all the diluting compartments of said unit, said membranes, spacers and gaskets having further aligned holes therein forming further manifold conduit means within each unit interconnecting the inlets of all the concentrating compartments of each unit and separately interconnecting the outlets of the concentrating compartments of said unit, the ports in said intermediate plates registering with said manifold conduit means, external disconnectable conduits connecting the manifold conduit means for the diluting compartment inlets and outlets of one unit with the manifold conduit means for the diluting compartment inlets and outlets of adjacent units, further external disconnectable conduits connecting the manifold conduit means for the concentrating com partment inlets and outlets of one unit with the manifold conduit means for the concentrating compartments of adjacent units, and electric current supplying means consisting of an anode at one end of the apparatus and a cathode at the other end of the apparatus for passing a direct current transversely through all the membranes and compartments.

4. An electrodialysis apparatus as claimed in claim 3 in which all'the intermediate plates, membranes, gaskets and intermembrane spacer members are of rectangular outline and in which the said external disconnectable connecting conduits connect with the intermediate plates along their outside edges at right angles to the manifold conduit means of each unit.

5. An electrodialysis apparatus as claimed in claim 3 in which all the intermediate plates, membranes, gaskets and inter-membrane spacer members are of similar parallelogramic outline with the parallelogramic shapes of adjacent units reversed so as to leave exposed portions of the end surfaces of the intermediate plates, and in which the said external disconnectable connecting con duits connect with the intermediate plates at such exposed end surfaces in line with the manifold conduit means of each unit.

6. An electrodialysis apparatus as claimed in claim 3 in which all the intermediate plates, membranes, gaskets and intermembrane spacer members are of similar generally rectangular outline and have unequal numbers of projections along opposite sides of the rectangle and in which the rectangles of adjacent units are inverted so as to leave exposed the end surfaces of the projections on the intermediate plates, and in which the said external disconnectable connecting conduits connect with the intermediate plates at such exposed end surfaces in line with the manifold conduit means of each unit.

7. An electrodialysis apparatus as claimed in claim 3 and subsidiary electrodes positioned in the rinse compartments of some intermediate plates.

References Cited in the file of this patent UNITED STATES PATENTS 2,694,680 Katz et al Nov. 16, 1954 2,758,083 Van Hock et al Aug. 7, 1956 2,758,965 Block et al Aug. 14, 1956 2,802,344 Witherell Aug.,13, 1957 OTHER REFERENCES Arnold et al.: The Industrial Chemist, July 1953, pp. 295498. 1 

1. AN ELECTRODIALYSIS APPARATUS COMPRISING A PLURALITY OF JUXTAPOSED MULTI-MEMBRANED UNITS EACH COMPRISING A FIRST INTERMEDIATE PLATE AT ONE END OF THE UNIT AND AN IDENTICAL INTERMEDIATE PLATE AT THE OTHER END OF THE UNIT RIGIDLY CONNECTED TO THE FIRST INTERMEDIATE PLATE, A PLURALITY OF PARALLEL ALTERNATING ANION-SELECTIVE AND CATIONSELECTIVE MEMBRANES BETWEEN SAID INTERMEDIATE PLATES, AT LEAST ONE GASKET AND ONE INTERMEMBRANE SPACER MEMBER BETWEEN EACH TWO MEMBRANES DEFINING BETWEEN SAID MEMBRANES ALTERNATE DILUTING AND CONCENTRATING COMPARTMENTS HAVING INLETS AND OUTLETS, ALL SAID MEMBRANES, GASKETS, INTERMEMBRANE SPACER MEMBERS AND INTERMEDIATE PLATES BEING PARALLEL TO EACH OTHER, SAID INTERMEDIATE PLATES BEING OF INSULATING MATERIAL AND OF THE SAME OUTLINE AS SAID MEMBRANES AND GASKETS, AND HAVING APERTURES THEREIN IN THE AREA THEREOF WHICH IS IN CONTACT WITH THE ACTIVE MEMBRANE SURFACES OF SAID MEMBRANES AND HAVING PORTS THEREIN, SAID MEMBRANES, GASKETS, AND SPACER MEMBERS HAVING ALIGNED HOLES THEREIN FORMING MANIFOLD CONDUIT MEANS SEPARATELY INTERCONNECTING THE INLETS OF ALL THE DILUTING COMPARTMENTS OF SAID UNIT AND SEPARATELY INTERCONNECTING THE OUTLETS OF ALL THE DILUTING COMPARTMENTS OF SAID UNIT, SAID MEMBRANES, SPACERS AND GASKETS HAVING FURTHER ALIGNED HOLES THEREIN FORMING FURTHER MANIFOLD CONDUIT MEANS WITHIN EACH UNIT INTERCONNECTING THE INLETS OF ALL THE CONCENTRATING COMPARTMENTS OF EACH UNIT AND SEPARATELY INTERCONNECTING THE OUTLETS OF THE CONCENTRATING COMPARTMENTS OF SAID UNIT, THE PORTS IN SAID INTERMEDIATE PLATES REGISTERING WITH SAID MANIFOLD CONDUIT MEANS, EXTERNAL DISCONNECTABLE CONDUITS CONNECTING THE MANIFOLD CONDUIT MEANS FOR THE DILUTING COMPARTMENT INLET AND OUTLETS OF ONE UNIT WITH THE MANIFOLD CONDUIT MEANS FOR THE DILUTING COMPARTMENT INLETS AND OUTLETS OF ADJACENT UNITS, FURTHER EXTERNAL DISCONNECTABLE CONDUITS CONNECTING THE MANIFOLD CONDUIT MEANS FOR THE CONCENTRATING COMPARTMENT INLETS AND OUTLETS OF ONE UNIT WITH THE MANIFOLD CONDUIT MEANS FOR THE CONCENTRATING COMPARTMENTS OF ADJACENT UNITS, AND ELECTRIC CURRENT SUPPLYING MEANS CONSISTING OF AN ANODE AT ONE END OF THE APPARATUS AND A CATHODE AT THE OTHER END OF THE APPARATUS FOR PASSING A DIRECT CURRENT TRANSVERSELY THROUGH ALL THE MEMBRANES AND COMPARTMENTS. 